Namespace QuadTree system

As suggested in https://github.com/CodingTrain/QuadTree/pull/37
We should move QuadTree classes into namespaces
As recommended by @shiffman, one of
- ```QuadTree```
- ```qt```
- ```qt5```

Review the current QuadTree classes and the suggestion in pull request 37 first. Choose one of the proposed namespaces—QuadTree, qt, or qt5—and define the scope of moving the classes; the work is done when the classes consistently use the chosen namespace.

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